The present invention relates generally to the field of nuclear power generation and in particular to a new and useful tube support bar for retaining and positioning water tubes within a nuclear steam generator.
Water tubes for nuclear steam generators are typically 0.5 to 0.75 inches in diameter with a nominal wall thickness of 0.045 inches. In the once-through steam generator design, the tube bundle consists of straight tubes. In a recirculating steam generator, depicted in FIG. 1, the tube bundle is made up of U-tubes.
In a pressurized water nuclear power station, steam generators, which are large heat exchangers, transfer heat, produced via nuclear reactions in the reactor core, from a primary water coolant to a secondary water coolant that drives the steam turbine. The primary coolant is pressurized, which allows the primary water coolant to be heated in the reactor core with little or no boiling. For example, in a light water reactor, the primary coolant is pressurized to about 2250 psia and heated to about 600 deg F. in the reactor core. From the reactor, the primary water coolant flows to a steam generator, where it transfers heat to the secondary coolant. In a U-tube, or recirculating steam generator, the primary coolant enters at the bottom of the steam generator, flows through tubes having an inverted U-shape transferring heat to the secondary coolant, and then exits at the bottom of the steam generator. The secondary coolant is pressurized only to a pressure below that of the primary side, and boils as it flows along the outside of the tubes, thereby producing the steam needed to drive the turbine.
Nuclear steam generators must be capable of handling large quantities of two-phase secondary coolant moving at high flow rates, and are therefore very large structures. For example, a nuclear U-tube steam generator can weigh more than 450 tons, with a diameter exceeding 12 feet and an overall length of greater than 70 feet. It may contain as many as 9,000 or more of the long, small diameter, thin-walled U-shaped tubes. For a general description of the characteristics of nuclear steam generators, the reader is referred to Chapters 47 and 52 of Steam/Its Generation and Use, 40th Edition, The Babcock & Wilcox Company, Barberton, Ohio, U.S.A., ©1992, the text of which is hereby incorporated by reference as though fully set forth herein.
Heat exchangers such as nuclear steam generators require tube restraints or supports, to position the tubes and to restrain the tubes against flow induced vibration forces. Tube support bars are therefore used in some nuclear steam generators is to keep the small diameter, thin wall heat transfer tubes in position and to prevent damage to the tubes due to vibration or external loads. In one tube support structure flat tube support bars are positioned at intervals along the tube bundle within the cylindrical shroud of the steam generator, forming lattice or tube support bar arrays. Each tube support bar array consists of two spaced rings that hold a latticework of crisscrossing flat bars between them. The flat bars, intersecting each other on their edges, form a diamond shape around each tube, thereby providing good vibration dampening yet allowing the steam-water mixture to flow through the tube bundle with minimal pressure drop.
One known type of lattice tube support bar array is manufactured by Babcock & Wilcox Canada Ltd. The lattice tube support bar array has a plurality of flat bars aligned parallel to one of two directions, for supporting the multiplicity of water tubes in the steam generator. When bars of different direction cross over each other, they form angles at bar intersections of 60° and 120°. Some of the bars, termed high-bars, provide most of the strength and rigidity of the array. Other smaller bars, termed low-bars, form a finer latticework that separates each tube. Low-bars comprise the majority of the bars in the array, and are about 1 inch high. Each low-bar is a unitary structure having flat sides made of a single material, typically stainless steel. High-bars, about 3 inches in height, are used about every 4 to 8 bars in the array, and have slots in their edges to permit bars arranged in the other direction to cross at the same level within a surrounding peripheral ring. The slots are typically 1 inch deep for low-bar intersections and 1½ inches deep for high-bar intersections. The high-bars are used to help position the low-bars within the array, and to transmit accumulated load to a peripheral heavy structural ring surrounding the bars. The peripheral heavy ring is connected to the outer shroud and shell of the steam generator, thereby conveying the support load to the shroud and shell.
As shown with exaggeration in FIG. 2, there is generally a gap between the heat exchanger tubes 90 and the low-bars 30, which is produced by the tolerance of the bar manufacture and is required for assembly. Similarly, gaps may exist between the heat exchanger tubes and the high-bars. Flow of steam and water past the tube induces vibrations which may not be effectively restrained due to the inherent gap. This in turn may reduce the tube life expectancy.
One known anti-vibration support is disclosed in U.S. Pat. No. 5,072,786, which describes a tube support bar design using a plurality of special hairpin springs. For a typical nuclear steam generator, this design requires the manufacture and assembly of a very, very large number of spring parts, so that it is difficult to apply the idea in practice. Therefore a new design for an anti-vibration tube support which reduces vibration, yet is easy to manufacture and install would be welcomed by industry.